I'm not sure if what this question is asking but ill be assuming that the average molecular mass is required. An assuming that the abundance of each isotope is 50% the average molecular mass is 69.82 amu.
Answer:
it holds reactant molecules in a good position for then to react
The mass number plays an important role for elements and their isotopes. Mass number comes from the addition of protons and neutrons (their weight). Isotopes are the elements, but with a different number of neutrons. So in turn, by subtracting the number of protons (atomic number) from the mass, you can find the number of neutrons.
